Tube

ABSTRACT

A tube of layered material consisting of a first layer and a second layer, for use in transporting and storage of hydrocarbons, wherein said first layer consists essentially of polyethylene or polypropylene, and wherein said second layer constitutes a barrier layer for reducing the diffusion outward through said tube of hydrocarbons transported or stored within said tube and comprises polyethylene or polypropylene in combination with any component selected from the group consisting of butyl rubber, polyamide, and polyester. The first and second layers of the tube are coextruded.

The invention relates to tubes and containers for transport and storageof petrol and hydrocarbons of a type similar to petrol, typically foruse in petrol station installations and other similar installationswhere petrol or. other hydrocarbons as i.a. kerosene, diesel oil, fueloil, natural gas, LPG, or the likeare stored in tanks and from theresupplied to e.g. the pumps in the filling station. Among the areas ofusage for these tubes the following may be mentioned, pipes for ventingand return of gases and other distributional areas.

In installations of the type referred to above it has been common to usegalvanized tubes each having a length of 6 meters, which means that thetubes have to be joined together. A joint always represents a threat ofleakage. This threat is heightened by the fact, that the weather changesduring the seasons of the year, gives effects from movements in theground which effects cannot be entirely avoided. It must of course bepointed out that even if the tubes are laid in the form of a continoushose of a plastic material, suitable for the application, the forcesfrom the ground. movements remain, but on account of the inherentproperties of the material the results of the existing forces may be.essentially eliminated. However the risk for leakage of hydrocarbons bydiffusion remains.

The leakage from the tubes generally used for this purpose mayapproximately be estimated to 0.4 g/meter and month, i.e. 2,5 1/meterand year. This estimate refers to polyethene tubes of the usual types.In addition to polyethene tubes, also tubes of glass fiber reinforcedplastic material is used.

The object of the invention is thus tubes or containers for transportand/or storing of petrol or hydrocarbons having similar properties,tubes manufactured from a material, said material being chemicallycompatible with said hydrocarbons and said material a reduced absorptionas to transported or stored hydrocarbons. Moreover the tubes arediffussion resistant to such an extent that they may be installed in theground with a higher degree of safety than the tubes existing up to now.

Generally speaking, most polyolefines may be used for transportingpetrol bearing in mind that most of the polyolefines show resistancetowards petrol to a greater or smaller extent. Among these arepolyethene and polypropylene, which materials are chemically inert andwhich also show favourable properties when used in connection withhydrocarbons. The most common use in this context is fuel tanks forinstallation in cars, said tanks for more than ten years having beenmanufactured from polyethene.

However, the matrix of polyethene (i.e. the molecular network of thematerial) does not belong to the most diffusion resistant plastics,since through this material diffuses and in the material is absorbedcertain amounts of solvents e.g. petrol. The thickness of themanufactured goods is of essential importance as are the sorroundingweather conditions, especially the temperature if the tube is installedabove ground and if the tube is installed in the ground the effectivefactors are essentially the same. That different types of plastics showdifferent permeability factors is well know in the art and this fact isused in various applications.

It is know in other fields of usage to coat tubes on the outside tohinder diffusion from the outside of gaseous elements or compounds intothe tube, conf. e.g. the coating on the outside of water-pipes in orderto hinder the admittance of oxygen to the circulating hot water in theradiator circuits.

It is also known to seal the inside of the fuel tanks in cars bylaquering them in order to stop the petrol fumes from diffusing. Thistype of treatment can-for natural reasons not be applied to continoustubes.

A gaseous diffusion resistant article is know from EP,A,2,0030091comprising 3 layers, of which one is the intermediate layer so chosen asto be an adhesive for both the layers and such that said layer alsocompensates for the relative coefficients of linear expansion betweenthe, intermediate layer and its adjoining layers. These tubes may beformed by co-extrusion. This tube is meant for central heating systemsand they thus have to be able to stand for a vide variety oftemperatures, much wider than the temperature range for the tubesaccording to the invention.

A number of documents showing examples of prior art in the relevantfield are the following JP,A,2035290, JP,A,3197034, JP,A,3224735.

The object of the invention is attained by the use of the tube accordingto the invention.

We have solved the above described problems by furnishing on the insideof a polyethene tube, said tube being of a conventional type for thispurpose and which tube is made from polyethene of the type MD-PD (mediumdensity polyethene), a barrier layer, compatible with both thepolyethene and the hydrocarbons which are to be transported or stored.As the barrier layer polyethene or polypropylene maybe used. This layeralso contains a second substance, which is chosen among polyamide,polyester and butyl rubber and which appear to be suitable substansesfor this purpose. Ethylen alcohol and vinyl alcohol, such as EVOH andEVAL, are also suitable for this purpose This second substance is eitherblended into the polyethene of the type MD-PD (medium densitypolyethene) or is grafted onto the polyethene.

One example of a barrier layer according to the invention may be used.The barrier layer chosen may be e.g. HD-polyethene containingbutylrubber having a composition where the butyl rubber component variesbetween 5 weight-% and 50 weight-% and a preferred amount is about 30weight-%, and the remainder essentially is HD-polyethene, which may becross-linked. Vi have found that when using percentages higher than 50-%processability problems occur and when using small additions the effectsof the butyl rubber are lessened and the addition of less than 5% doesnot give a satisfactory action. This same proportions have shown to beworking with the other substances mentioned above as the secondsubstances mentioned above.

One important aspect of this invention is that there is no 3 necessityof providing any form of extra layer or any kind of glue in between thetwo layers, i.e. the main layer and the barrier layer. The two layersmay simply be co-extruded as they are compatible with each other.

The barrier material is an expensive material, which means that the useof the material is restricted to the amount necessary for obtaining theset goals. According to our opinion using a barrier layer having athickness of 10-15% of the total material will decrease the diffusion toone tenth of the value usually encountered for used materials today. Wehave found that for practical reasons it is hard to make, the barrierlayer thinner than 0.3 mm—a preferred thickness of the layer is 1 to 2mm. To make a thicker layer than this does not add anything to theproperties of the barrier layer and doing so would also intrude on theavailable space in the tube. The comparatively thin inner laster willnot change the price drastically and the favourable experiences made inusing polyethene in this context in a long term perspective are notchanged since we are not exchanging material in the polyethene tube butare adding a small amount of material on the inside of the same. Ifinstead we had chosen to put a layer on the outside of the tube allmechanical properties as as tensile strength of tube fittings i.a. wouldhave to be tested and re-evaluated over a long period of time. Byproceeding as above we are able without further testing and developmentuse the fittings and such and moreover be certain that all of thestandard values for the properties of the polyethene as a pressure tubewill remain.

Transport of fluids and liquids through tubes gives rise to staticelectricity which always has to be led away. Within the art it is wellknown to add carbon black to the plastic material in order to make theplastic material conducting. The amount added depending on the qualityof the carbon black and the intended area of use, i.e. the degree ofconductivity which is required in order to obtain the desired results. Asuitible value of resistivity in order to avoid static electricit seemsto in the range of 5 to 500 ohm/cm. This component may be added to themain layer.

In order to additionally improve the safety in connection with this typeof tube installations a type of leakage detector may be arrangedlengthwise, as a narrow strip, on the tube in the form of conductinglayer, which would allow the measuring of the amount of resistanse inthe strip in a manner well know to the man skilled in the art. Themechanism behind this is that if the plastic material absorbs solventsit will swell and the mutual distances between the particles of carbonblack will increase and as a consequence the resistance in the strip,which will give a clear indication as to solvents leaking from the tube.

To co-extrude this strip when forming the tube is extremely practicalsince the installation then will consist of one tube only instead ofhaving an extra tube near the first one as today.

Comparative tests have been performed with tree types of polyethenetubes having diameters of 66 mm. The tubes consisting of a standard tube(1) of the type SA 1-1-2305-5, a tube (2) having a barrier layer ofbutyl-grafted HD-polyethene and a tube (3) having a barrier layercontaining HD-polyethene blended with a thermoplastic polyester.

The tubes were filled with synthetic petrol in accordance with ABSvenska Anläggningsprovnings Technical Instruction TA 14-01. The petrolconsisted of iso-octane 50%, toluene 30% and orto-xylene 20% allpercentages by volume.

The tubes were weighed and burried in a box filled with gravel, exceptfor the standard tube. The box measured 30×30×200 mm. The rest of thetubes, three, were kept in air. All test objects were kept at-roomtemperature. After 30 days following results were recorded:

Weight loss 30 days grams/meter in Sample gravel tube in air 1 — 0.00 20.00 0.05 3 0.00 0,00

The tubes having a barrier layer of butyl-grafted HD-PD have also beentested as to pressure resistance and resistance in regards to chemicals.The tested tubes had a diameter of 66 mm and a wall-thickness of 5 m forthe outer layer and of 1,2 mm for the barrier layer.

In pressure test (80° C., 170 hours according too SS 3362.7 and 0.8) ontubes having a lenght of 400 mm following results were recorded.

Mean outer Total Thickness Time of diameter min max P rupture Sample mmmm mm MPa MPa hours 1 66.4 6.50 7.00 4.6 0.749 >170 2 66.4 6.40 7.10 4.60.749 >170 3 66.4 6.40 7.10 4.6 0.749 >170 4 66.4 6.30 7.10 4.60.749 >170 5 66.4 6.50 7.00 4.6 0.749 >170

The test pressure (P) is calculated for a wall thickness of 5.00 mmwhich relates to the part of the wall thickness of the tube which is notthe barrier layer.

Axial deformation was measure on 3 samples and following results wererecorded.

Axial deformation Change of length Mean value −1.1 Sample in % Demande±3 1 −1.0 2 −1.0 3 −1.2

A tensile test was also performed on 3 samples.

Tensile test +23° C.

Rate of pulling 25 mm/min Sample kN 1 9.9 2 9.7

A tensile test was also performed on pieces of tube material which hadbeen kept in a temperature of +80° C. The pieces were mill to athickness of 1 mm having the inside of the tube wall unharmed. Testspecimens were cut as a test bar according to DIN 53455. Four specimensware kept for 14 days at −23° C. in a liquid consisting of one partsynthetic petrol (as below) and one part methanol.

The synthetic petrol consisted of:

60 parts iso-octane

5 parts benzene

20 parts toluene

15 parts of o-xylene

The specimens were weighed immediatly before and after the aboveprocedure.

Weight Yield point Elongation at Specimen change % MPa yield point %Non-treated 1 — 14.7 2.0 2 — 15.1 1.4 3 — 15.3 1.6 4 — 15.2 1.4 5 — 14.71.6 Mean value — 15.0 1.6 Treated 6 15.1 13.5 4.0 7 15.0 13.3 3.8 8 15.013.4 3.8 9 15.2 13.3 3.8 10  15.0 13.0 4.0 Mean value 15.1 13.3 3.9Change in % +15.1 −11 +144

Pressure test

A tube was supplied with fittings and then placed in a water bath at 20°C. and a pressure of 25 bar was applied. After one hour the test wasinterrupted and the tube had not burst.

The tube described above was made by co-extrusion of the two layers. Itis of course not quite impossible to apply the barrier layer onto theinside of the tube after the tube has been extruded.

But since the tubes are to be produced in the form of continuous tubesco-extrusion seems to be the most appropriate way of manufacture.

In the manufacture of containers for storage or parts having biggerdimensions than the herein described tubes the barrier layer may ofcourse be applied to the finished articles.

The invention has been described with referens to testsamples. It is ofcourse possible to modify the invention within the scope of the appendedclaims.

What is claimed is:
 1. A tube of layered material consisting of a firstlayer and a second layer characterized in that said first layer consistsessentially of a material selected from the group consisting ofpolyethylene and polypropylene, and said second layer, constituting abarrier layer for reducing the diffusion outward through said tube ofhydrocarbons transported or stored within said tube, comprises amaterial selected from the group consisting of polyethylene andpolypropylene in combination with any of the components selected fromthe group of components consisting of butyl rubber, polyamide andpolyester, said first layer and said second layer being coextruded witheach other.
 2. Tube according to claim 1, characterized in that thesecond layer has a thickness of 1 mm to 2 mm.
 3. Tube according to claim1, characterized in that the second layer consists of a blend ofpolyethylene or polypropylene and one of said components selected fromsaid group of components.
 4. Tube according to claim 1, characterized inthat the second layer is placed on the inside wall of the tube.
 5. Tubeaccording to claim 1, characterized in that the second layer comprises ablend of polyethylene or polypropylene, and one of said componentsselected from said group of components, whereby the added amount of saidone of said components is in a range from 5 to 50 weight-% as related tothe weight of the polyethylene or polypropylene.
 6. Tube according toclaim 1, characterized in that the second layer comprises a blend ofpolyethylene or polypropylene, and butyl rubber, whereby the addedamount of butyl rubber is 30 weight-% as related to the weight of thepolyethylene or polypropylene.
 7. Tube according to claim 1,characterized in that an amount of an electrically conducting substanceis added to said first layer in order to avoid static electricitybuilding up.
 8. Tube according to claim 1, characterized in that thesecond layer has a thickness of 10 to 15% of the total thickness of thetube.
 9. Tube according to claim 1, characterized by an electricallyconducting strip of plastic being arranged along the total lenght of thetube.
 10. Tube according to claim 1, characterized in said first layerconsists essentially of medium density polyethylene.
 11. Tube accordingto claim 1, characterized in said second layer comprises high densitypolyethylene.
 12. Tube according to claim 1, characterized in that saidsecond layer comprises polyethylene in combination with polyamide.